What is silica?
Silica is a chemical compound consisting of the element silicon, which is a semiconductor used in electronics and computers, and the element oxygen. Chemically, silica is silicon dioxide with the formula SiO2, but this simple formula doesn‘t tell but part of the story because silica is polymerized or has crystal structure. Silica is one of the most common minerals on earth and is found in most rocks, e.g., quartz and amorphous opals. Silica is a source of the element silicon for the human body.
The origin of geothermal silica
Geothermal silica is formed when fresh water or sea water seeps down into the ground through rocks and natural fissures, and comes into contact with very hot rocks at great depths. The resulting hot water at high temperatures and high pressure dissolves minerals like silica from the rocks. The hot water including dissolved minerals is commonly called geothermal fluid. This hot fluid rises up, since it is less dense than colder fluid, where it cools and sinks back down again, setting up a convective hydrothermal system. The fluid that exits from the boreholes at the surface is almost always a two phase mixture of steam and liquid, the liquid commonly being much hotter than 200°C.
This hot geothermal fluid is saturated with dissolved silica which is in the form of silicic acid, and as soon as the fluid cools, it becomes supersaturated with respect to the silicic acid. When that happens, the silicic acid polymerizes into tiny polymers that are constantly forming and dissolving again. Under certain thermodynamically conditions these tiny polymers become stable and start to get larger, both due to further polymerization and molecular deposition of silicic acid molecules. This process forms extremely small silica particles that then aggregate into even bigger particles. The final size of the particles depends heavily on the salinity of the geothermal fluid and other variables like the cooling profile. Silica in this form is called precipitated silica and geothermal silica is the only natural source of precipitated silica.